Brazing dissimilar metal members



. 1963 R. P. SKINNER ETAI. 3,

BRAZING DISSIMILAR METAL MEMBERS Filed 001;. 23, 1958 INVENTORS RANSOM P. SKINNER RICHARD M.POORMAN ATTOR EV United States Patent 3,105,293 BRAZING DISSKMILAR METAL MEMBERS Ransom P. Skinner and Richard M. Poorman, Indianapolis, Ind, assignors to Union Carbide Corporation, a corporation of New York Filed Oct. 23, 1958, Ser. No. 769,224 2 Claims. (Cl. 29-474A) This invention relates to the production of a brazed joint between dissimilar metal members, and more particularly to the production of a brazed lap joint between a first metallic member and a second dissimilar metallic member, the latter having a higher melting temperature than the first metallic member, as for example, a joint between aluminum and stainless steel.

Aluminum is often desirable as a construction material because of its properties of light weight, high thermal conductivity, and low electrical resistance. In many of its applications, however, undesirable results are obtained when joints must be made between aluminum and other metals. Such connections, for example, are required in the refrigeration industry when aluminum evaporator coils are joined to steel vessels. They are also required in oxygen supply systems for high altitude breathing, when a double walled aluminum liquid oxygen storage container may have stainless steel tubing positioned between the inner and outer vessels. In each case the dissimilar metal joint must be leaktight and must retain its strength at low temperatures. Joints between aluminum and stainless steel or iron made by the prior art methods are unsatisfactory because of frequent failure caused by embrittlement of the aluminum-iron intermetalic compounds.

Many solutions to the dissimilar metal-joint problem have been suggested. One proposed solution consists of placing an aluminum coating of controlled thickness on the steel article and then casting or brazing the aluminum article to the aluminum coating. Such method provides joints which may be useful at temperatures below about 500 F. under relatively low flexing conditions. However, such joints fail under more rigorous conditions due to the presence of the brittle aluminum-iron alloy at the joint interface.

Another proposed solution to the joint embrittlement problem utilizes a silver intermediate layer. In such method, the steel is plated with a 0.00005 to 0.0005 inch thick layer of silver. The aluminum is then placed in contact with the silver plating and the joint is formed by electrical resistance welding combined with high pressure. A portion of the silver diffuses into and alloys with the aluminum to provide a strong aluminum-silver-iron bond with minimum formation of brittle aluminum-iron alloys. However, in such process, very careful control is required to prevent all of the extremely thin layer of silver from diffusing into the aluminum and allowing a substantial amount of brittle aluminum-iron alloy to be formed. Another serious problem in such method is the extreme difficulty in a resistance welding system of arriving at an ideal joining temperature for all the metals, since aluminum, silver and the other joint materials all have different melting points. For example, aluminum has a relatively low melting point (about 1200 F.) compared to stainless steel (above 1900 F.) and silver (about 1760" F.) Another problem which must be faced is the different coefficients of thermal expansion, aluminum having a higher coetficient than stainless steel. Resistance welding is also limited to parts wherein large concentrated pressures can be applied and backed up. his impractical to use such a resistance welding process for joining slender, thin-walled tubes because they cannot "Ice Withstand the required high pressures nor can back-up electrodes be inserted inside such tubes.

Still another solution has been proposed wherein an article with a relatively thick (up to 0.032 inch) coating of silver is electric-arc welded to aluminum using aluminum welding rod. Such method requires extremely high temperatures in the joint area and is very critical in operation to attain proper penetration of the weld head into the joint area especially when thin-walled (0.004 to 0.010 inch thick) tubular joints are being made. Another disadvantage of such method is that it does not permit the formation of true lap joints with adequate shear area. Furthermore, relatively thick silver coatings of satisfactory quality are difiicult to obtain and are expensive to apply by standard techniques such as electroplating.

A further prior art attempt to produce joints between aluminum and dissimilar metals such as copper involves tinning of the copper with a silver solder prior to aluminum brazing it to aluminum. These tinned coats were probably of the order of 0.0005 inch thick and were used only to improve the wetting action during brazing by protecting the copper from oxidation. The resulting joint had low peel strength and was relatively brittle due to the complete and early solution of the silver into the aluminum with subsequent formation of brittle aluminumcopper alloys.

One object of the present invention is to provide a highly efiicient process for brazing together a first metallic member such as aluminum and a second metallic member such as stainless steel, which process provides a leaktight joint and avoids the formation of aluminum-iron intermetallic compounds which cause weld failure by embrittlement.

' Another object is to provide a process for welding together thin tubes of aluminum and stainless steel without requiring the application of high pressures which would distort the thin walls of the dissimilar metal tubes.

A still further object of the present invention is to provide a composite brazed assembly of aluminum and stainless steel with a non-brittle leak-tight joint capable of withstanding high flexing and severe temperature conditions ranging from about l91 C. to above-atmospheric temperatures.

These and other objects of the invention will in part be obvious and in part become apparent from the ensuing description and accompanying drawings in which:

FIG. 1 is a view of a longitudinal cross-section through a lapping assembly including two dissimilar metal tubes just before lap brazing such tubes together according to one form of the present invention; and

FIG, 2 is a view of an enlarged longitudinal crosssection through the completed dissimilar metal lap brazed joint.

According to the present invention, a novel process is provided for lap brazing together a first metallic member having a first melting temperature and provided with a lappable surface of given contour, auda second dissimilar metallic member having a second melting temperature higher than the first melting temperature. The second member is provided with a lappa'ble surface of contour and size corresponding to the :given contour of the first metallic member lappable surface. The first member may for example, be constructed of aluminum and the second member formed of stainless steel. The lappable surface of the latter member is first coated with a compatible intermediate bonding material such as silver, which has a third melting temperature between the melting temperatures of the aluminum and stainless steel members. The intermediate bonding material coating is preferably at least about 0.002 inch thick for reasons to be discussed herein after. The lappable surface of first metallic member and 3 the coated lappable surface of the second metallic member are then positioned in lapping relationship and a brazable alloy containing the first metal of the first metallic member as a major constituent is provided adjacent such surtaces. This brazable alloy can be positioned directly between and in contiguous relation to the lappable surfaces of the first member and the coated second member. Alternatively it could be positioned in adjoining relation to these surfaces and be caused to flow into contiguous relation therewith upon application of sufiicient heat. The brazable alloy has a fourth melting temperature below the melting temperatures of the other three materials, and may, for example, be an aluminum brazing alloy. The lapping assembly is then heated only sufiiciently until the brazable alloy has melted and a desired amount of diffusion has occurred between the brazable alloy and the intermediate bonding material as well as between the brazable alloy and the first metallic member. In this manner a ductile leak-tight joint is formed substantially free of brittle alloys and in which the first metallic member is in contact only with the brazable alloy, and the second metallic member is in contact with insufi'icient amounts of the first member metal to form brittle intermetallic alloys. It is preferable that the second metallic member be in contact only with the intermediate bonding material.

The present invention also contemplates a composite lap brazed assembly comprising the aforedescribed first and second metallic members with an inter-mediate bonding material coating on a lappable surface of the second member having an initial thickness before brazing of preferably at least 0.002 inch. The two lappable surfaces are connected by a brazed leak-tight, ductile lap joint including a transition alloy layer contiguous to the substantially first metal-free remaining intermediate bonding material coating and comprising said intermediate bonding material and the previously described brazable alloy, and a layer of the brazable alloy contiguous on one side to the transition alloy layer and on the other side to the first metallic member lappable surface.

The aforedescribed process is particularly suited [for joining dissimilar metal members in the form of tubes because it avoids the high pressure used in conjunction with resistance-welding techniques, which could distort thin metal tubes. When aluminum and stainless steel tubes are to be joined according to the present invention, the aluminum tube is preferably positioned around the outside of the joint. This is because aluminum has a higher coefiicient of expansion than the second dissimilar metals used in thejoint, and its contraction after the joint cools tends to strengthen the brazed joint. When the subject joints are used for low temperature service as at liquid oxygen temperature, the additional contraction of aluminum aids in maintaining a strong leak-tight joint.

One advantage of the present process for joining dissimilar metals is that the necessary heat is applied by brazing, for example, using a hand torch, a furnace, or a salt bath. Brazing is extremely versatile and may be used with many joint shapes which are physically difllcult to form by other methods such as resistance welding. Also the lower temperatures required for melting the brazable alloy do not produce the extreme thermal conditions characteristic of prior art electric arc methods for joining aluminum to dissimilar metals. Joint distortion and penetration of the silver is thus kept at a minimum.

The present process also avoids the formation of brittle aluminum-ferrous or copper alloys; instead a strong bond is obtained with a transition alloy layer separating the aluminum brazed alloy layer contiguous to the aluminum member and the substantially aluminum-free silver intermediate bonding material layer contiguous to the second dissimilar metal member. Thus, the aluminum brazed alloy'layer in direct Contact with the aluminum member contains no second dissimilar metal molecules and the silver layer in direct contact with the second dissimilar metal member contains substantially no aluminum mole- 4 cules, either of which could cause formation of undesirable alurninum-second dissimilar metal alloys.

To obtain a high peel strength for the joint, it is essential that a distinct substantially aluminum-free silver layer remain next to the dissimilar second metal after the jointis completed. Accordingly, it has been found that the intermediate bonding material layer must have an initial thickness before brazing of at least about 0.002 inch. When the coating thickness is less than this value, there is a tendency for such coating to completely diifuse into the brazing material and allow substantial amounts of a brittle aluminum-dissimilar metal intermetallic alloy to be formed which renders the joint unsatisfactory. This is particularly important when aluminum-ferrous metal and aluminum-copper joints are produced. An intermediate bonding material coating thickness of about 0.003 inch to 0.007 inch is preferred, this range representing an optimum balance between joint strength, joint size, and joint production costs. It will be recognized that as the intermediatebonding material thickness increases, the joint will become progressively larger and bulkier, and joint production costs will also increase. It is practical, however, to form dissimilar metal joints according to the present invention by using intermediate bonding material coatings having thicknesses as large as 0.010 inch.

Referring now to the drawings and specifically to FIG. 1, the outer walls of an end of the second metallic member, for example in the form of stainless steel hollow tube 10, constitute a lap-pable surface. Tube 10 may be formed from a member of the group consisting of ferrous metal, [ferrous alloys, nickel, nickel alloys, copper and copper alloys, as these materials are suitable as the second metal from which the second metallic member is provided. The second member lappable surface is coated with an intermediate bonding material such as silver or silver alloy to form a coating 11 of desired thickness and length. Various coating methods well known to those skilled in the art maybe used, although electroplating is convenient and preferred for pure silver coatings. Metal spraying, hot dipping and soldering techniques can also be used, especially with silver alloy coatings.

A first metallic member, for example, in the form of aluminum hollow tube 112 is provided and preferably has an inner wall diameter which is slightly larger than the outer wall diameter of coated stainless steel hollow tube 10 so that the latter may be slipped in and concentrically aligned in the end of aluminum hollow tube 12 with a narrow annular clearance space therebetween. In this way the first and second metallic members are positioned in a lapping relationship which permits the formation of e a true lap joint having a large shear area and hence increased joint strength. It may also be desirable to coat either or both of the lappable surfaces with a suitable brazing flux before joint formation, as will be understood by those skilled in the art.

A layer of a grazable alloy 13 containing the first metal of the first metallic member as its major constituent, e.g. aluminum brazing alloy, may be placed in and substantially fill the annular space between the silver coated stainless steel tube and the surrounding aluminum tube. The brazable alloy has a melting temperature below the melting temperature of the other three materials, namely, the stainless steel and aluminum tubes and the silver coating. In the case of aluminum brazing alloys, aluminum is the major'constituent and relatively small percentages of other elements such as silicon, copper, iron, zinc, magnesium and manganese may also be present. Other brazable alloys having dilferent major constituents could also contain traces of at least some of these elements as long as they do not have a detrimental effect on the brazed 7 joint. The brazable alloys may be used in any convenient form such as wire, rod, sheet or powder although the form is usually selected on the basis of the type of joint to be brazed. For example, when making a tubular joint such as illustrated in FIG. 2, the rod form of brazable alloy has been found suitable and may, for example, be wrapped around the silver coated stainless steel tube end prior to insertion of the latter in the aluminum tube, thus forming layer 13. It may also be wrapped around the coated stainless steel tube near the lappable surfaces and flow into contiguous relation upon application of heat.

The lapping assembly is then uniformly heated by conventional means such as hand torch 14 until the leaktight, ductile joint illustrated in FIG. 2 is formed. As heat is applied to the materials, the aluminum brazing alloy melts first since it has the lowest melting point of such materials, e.g. 1070-1080 F. As it melts, the aluminum brazing alloy diffuses into the contiguous layer of silver on one side and [forms a bond with the contiguous inner Wall of the aluminum tube on the opposite side. The silver coating then begins to melt and a transition alloy layer 15 is formed between the remaining aluminum brazing alloy layer 16 and the remaining silver coating 17, layer 15 containing both the aluminum brazing alloy material and the silver coating materal. The concentration of aluminum brazing material is 100% at the interface of the aluminum brazing layer \16 and the transition alloy layer 15 and gradually decreases along the line 22 moving transversely through the transition alloy layer away from the aluminum brazing alloy layer until it reaches a concentration of essentially 0% at the interface of the transition alloy layer 15 and silver layer 17. Conversely, the concentration of silver is substantially 100% at the interface of the transition alloy layer 15 and the silver layer '17 and gradually decreases along line 2-2 moving transversely through the transition alloy layer away from the silver layer until it reaches a concentration of essentially 0% at the interface of the transition alloy layer 15 and the aluminum brazing layer 16. It can thus be seen that transition alloy layer 15 separates aluminum brazing alloy layer 16 from silver coating 17 and prevents the diffusion of substantial amounts of ferrous molecules into the aluminum brazing layer and aluminum molecules into the remaining silver coating. The transition alloy layer 15 itself is composed mainly of aluminum and silver alloys which are very ductile and leaktight.

The unique advantages of the present invention are further illustrated by the following examples:

EXAMPLE I Formation of Aluminum-Stainless Steel Tubular Joint A type 304 stainless steel tube 1 /2 inches CD. was coated externally by hot-dipping with a layer of silver solder about 0.003 inch thick, as determined by microscopic examination. The silver solder had a composition of 85% silver and 15% zinc (by weight). A thin coating of brazing flux was then applied to this silver solder coating and an aluminum brazing alloy rod of about inch diameter was wrapped around the joint area. The aluminum brazing rod had an approximate melting range of 1070-1080 F. and contained as alloying agents approximately 11.0-13.0% by weight silicon, 0.30% copper, 0.80% iron, 0.20% zinc, 0.10% magnesium, and 0.15% manganese. The stainless steel tube thus prepared was slipped into an aluminum tube, the joint end of which flared from 1 /2 inches to about 1%; inches LD. It is common brazing practice to flare the outer tube of a tubular joint to allow escape of flux during the brazing operation. The interior of the aluminum tube WS also previously thinly coated with brazing flux. Heat was applied externally to the joint area of the aluminum tube by means of an oxygen-acetylene torch until the brazing rod melted and filled the gap between the silver solder-coated stainless steel and the aluminum surfaces. The resulting joint successfully withstood leak and physical deformation tests.

6 EXAMPLE n Formation of Aluminum-Copper Lap Joint A copper tube 7 inch CD. was machined externally on one end to form a slip fit through a inch diameter opening in a 0.042 inch thick aluminum vessel. The length of the machined portion was about 0.050 inch. The copper tube was coated externally by hot-dipping with a layer of silver solder about 0.003 inch thick. The silver solder had a composition of silver and 15% zinc. A thin coating of suitable brazing flux was then applied to the silver solder coating, and the so-prepared copper tube was slipped into the opening in the aluminum vessel. The inner Walls of the opening were also previously coated with brazing flux. An aluminum brazing rod about li inch diameter and having the same composition as the rod described in Example I, was Wrapped around the outside of the copper tube adjacent to the walls of the opening in the aluminum vessel. Heat was applied externally to the joint area by means of an oxygenacetylene torch until the brazing rod melted and filled the gap between the silver solder-coated copper and the aluminum. The resulting joint successfully withstood leak and physical deformation tests.

Tensile strength tests of aluminum brazing-silver-stainless steel tubular joints prepared by the process of the present invention indicated that the joints were stronger than the stainless steel base metal since tensile failure occurred first in that member. Aluminum-stainless steel joints produced by prior art methods may be easily pulled apart at the joint due to the presence of the brittle aluminum-iron intermediate compound.

Although the invention has been described and illustrated specifically for the joining of dissimilar metal tubes, it is to be understood that it is equally suitable for the lap brazing of dissimilar metal members having other shapes. The basic requirement is that the first metallic member must have a lappable surface of given contour, and the second dissimilar metallic member must have a lappable surface of contour and size corresponding to the contour of the first member lappable surface. For example, the two members may have flat lappable surfaces.

It is to be understood that certain modifications may be made to the specific embodiments of the invention as disclosed herein, without departing from the scope of such invention. Also, some features may be practiced without utilizing other elements of the invention, all within the aforementioned scope.

What is claimed is:

l. A process for lap brazing together an aluminum tube and a stainless steel tube comprising the steps of coating one end of the stainless steel tube with a layer of silver intermediate bonding material having a thickness of from about 0.002 to about 0.010 inch, the silver coated stainless steel tube having an outer wall diameter slightly smaller than the inner Wall diameter of said aluminum tube so as to be receivable therein with a narrow annular space therebetween; applying a thin coating of a brazing flux to the silver coating on said stainless steel tube; positioning the silver coated end of said stainless steel tube inside an end of said aluminum tube; providing an aluminum brazing alloy adjacent the silver coated end of said stainless steel tube, the brazing alloy having a melting temperature below that of the aluminum and stainless steel tubes as well as the silver intermediate bonding material; heating the portion of said silver coated stainless steel tube inside the end of said aluminum tube only sufficiently until said aluminum brazing alloy has melted and a desired amount of diffusion has occurred between the aluminum brazing alloy and the silver intermediate bonding material as well as between the aluminum brazing alloy and the aluminum tube inner wall so as to form a ductile leaktight joint free of brittle alloys wherein the aluminum tube inner wall is in contact only with the aluminum brazing alloy and the 7 1 stainless steel tube outer wall is in contact primarily with the silver intermediate bonding material.

2. A process for lap brazing together an aluminum tube and a second tube constructed from a metal taken from the class consisting of ferrous metals, ferrous alloys, nickel, nickel alloys, copper and copper alloys comprising the steps of coating one end of the second tube with a layer of intermediate bonding material taken from the class consisting of silver and silver alloys, said coating having a thickness of from about 0.002' to about 0.010 inch, the socoated second tube having an outer wall diameter slightly smaller than the inner wall diameter of said aluminum tube so as to be receivable therein with a narrow annular space therebetween; applying a thin coating of brazing flux to the coating on said second tube; positioning the coated end of said second tube inside the end of said aluminum tube; providing an aluminum brazing alloy adjacent the coated end of said second tube, the brazing alloy having a melting temperature below that of the aluminum and second tube as Well as the intermediate bonding material; heating the overlapping portions of said second tube and said aluminum tube only sufficiently until said aluminum brazing alloy has melted and a desired amount of diffusion has occurred between the aluminum brazing .alloy and the aluminum tube inner wall so as to form a ductile, leaktight joint free of brittle alloys wherein the aluminum tube inner wall is in contact only with the aluminum brazing alloy and the second tube outer wall is in contact primarily with the intermediate bonding material.

References Cited in the file of this patent UNITED STATES PATENTS 

2. A PROCESS FOR LAP BRAZING TOGETHER AN ALUMINUM TUBE AND A SECOND TUBE CONSTRUCTED FROM A METAL TAKEN FROM THE CLASS CONSISTING OF FERROUS METALS, FERROUS ALLOYS, NICKEL, NICKEL ALLOYS, COPPER AND COPPER ALLOYS COMPRISING THE STEPS OF COATING ONE END OF THE SECOND TUBE WITH A LAYER OF INTERMEDIATE BONDING MATERIAL TAKEN FROM THE CLASS CONSISTING OF SILVER AND SILVER ALLOYS, SAID COATING HAVING A THICKNESS OF FROM ABOUT 0.002 TO ABOUT 0.010 INCH, THE SOCOATED SECOND TUBE HAVING AN OUTER WALL DIAMETER SLIGHTLY SMALLER THAN THE INNER WALL DIAMETER OF SAD ALUMINUM TUBE SO AS TO BE RECEIVABLE THEREIN WITH A NARROW ANNULAR 